I have a main website, and a couple of subdomains; I am trying to validate my main site but when I go into the Wordpress admin area and add the license code into the field and hit 'enter' the screen goes blank and nothing happens.

I'm worried as I can now no longer access my pages in cornerstone and I need to work on the website.

Thank you for writing in, Your version of X (5.1.1) and Cornerstone (3.1.6) is not compatible, this is more likely the culprit of your issues. Please update your X and Cornerstone to the latest version (6.2.5 and 3.2.5 respectively), you can check what are the latest compatible versions here.